When the brakes of a vehicle are applied, a braking force between the wheel and the road surface is generated that is dependent upon various parameters including the road surface condition and the amount of slip between the wheel and the road surface. For a given road surface, the force between the wheel and the road surface increases with increasing slip values to a peak force occurring at a critical wheel slip value. As the value of wheel slip increases beyond the critical slip value, the force between the wheel and the road surface decreases. Stable braking results when the slip value is equal to or less than the critical slip value. However, when the slip value becomes greater than the critical slip value, braking becomes unstable resulting in sudden wheel lockup, reduced vehicle stopping distance and a deterioration in the lateral stability of the vehicle.
Numerous wheel lock control systems have been proposed to prevent the wheels from locking up while being braked. Typically, these systems prevent the wheels from locking by releasing the applied brake pressure when an incipient wheel lockup condition is sensed. One criterion that is used to sense an incipient wheel lockup condition is excessive wheel deceleration. After release of the brake pressure, the wheel deceleration ceases and the wheels then accelerate toward vehicle speed. When the wheel speed has substantially recovered, brake pressure is reapplied. One criterion that is typically used to sense recovery is wheel acceleration falling below a specified low value. Reapplication of brake pressure results in the wheel again approaching lockup and the cycle is repeated. This form of wheel lock control system results in a rapid cycling of the brake pressure and the wheel slip value around the critical slip value to prevent wheel lockup. Ten Hertz is a typical cycling frequency. Brake pressure modulators of various types have been used to cycle and limit the brake pressure at the vehicle wheel brakes to accomplish this.